The invention relates to semiconductor manufacturing and more specifically to a method and apparatus for controlling the delivery of slurry through a polishing pad in a chemical mechanical polishing (CMP) tool.
A flat disk or xe2x80x9cwaferxe2x80x9d of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough. The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, the material layers (deposited thin film layers usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness.
Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing process steps that create the integrated circuitry or interconnects on the wafer. A considerable amount of effort in the manufacturing of modern complex, high density multilevel interconnects is devoted to the planarization of the individual layers of the interconnect structure. Nonplanar surfaces create poor optical resolution of subsequent photolithography processing steps. Poor optical resolution prohibits the printing of high-density lines. Another problem with nonplanar surface topography is the step coverage of subsequent metalization layers. If a step height is too large there is a serious danger that open circuits will be created. Planar interconnect surface layers are required in the fabrication of modem high-density integrated circuits. To this end, chemical-mechanical polishing (CMP) tools have been developed to provide controlled planarization of both structured and unstructured wafers.
CMP consists of a chemical process and a mechanical process acting together, for example, to reduce height variations across a dielectric region, clear metal deposits in damascene processes or remove excess oxide in shallow trench isolation fabrication. The chemical-mechanical process is achieved with a liquid medium containing chemicals and abrasive particles (commonly referred to as slurry) that react with the front surface of the wafer while it is mechanically stressed during the planarization process.
In a conventional CMP tool for planarizing a wafer, a wafer is secured in a carrier connected to a shaft. Pressure is exerted on the back surface of the wafer by the carrier in order to press the front surface of the wafer against the polishing pad in the presence of slurry. The wafer and/or polishing pad are then moved in relation to each other via motor(s) connected to the shaft and/or platen in order to remove material in a planar manner from the front surface of the wafer. Various combinations of motions are known for moving the wafer and polishing pad in relation to each other. For example, the wafer is commonly rotated or held stationary and the polishing pad is moved in either a linear, rotational or orbital manner.
A common problem in CMP is for the wafer to polish in a nonplanar manner. The wafer typically has a xe2x80x9cbull""s-eyexe2x80x9d pattern with the center of the wafer being polishing either faster or slower than the circumference. The polishing rate tends to be uniform within concentric bands, but not across the entire surface of the wafer. Numerous attempts have been made to remedy this problem with only partial success. This problem has recently worsened as some of the slurries used to planarize wafers with copper thin films result in nonuniform material removal with limited process control.
One attempted solution to solve the problem when the center is being polished too slowly is to move the edge of the wafer over the edge of the polishing pad. This will slow the removal rate of material at the edge of the wafer to more closely match the removal rate at the center of the wafer. This solution is relatively inexpensive, but has several problems. One problem is that this solution is not able to compensate for the center fast situation. In addition, front-reference carriers (those supporting the wafer by air or a membrane) tend to break or lose control of the wafer when the wafer is placed over the edge of the polishing pad. Another problem is that this approach has minimal flexibility in fine tuning the removal rate over the entire surface of the wafer.
Another attempted solution is to use a multizone carrier. Multizone carriers have a central zone and one or more concentric zones for altering the polishing rate for corresponding concentric zones on the wafer. Each of the zones in the carrier may be configured to apply an individually controllable pressure on the back surface of the wafer. In this way, concentric bands that are polishing too quickly or too slowly on the front surface of the wafer may receive a correcting lower or higher pressure on the back surface of the wafer by the multizone carrier. This approach adds more flexibility to the process, but also adds a great deal of expense and complexity to the process.
What is needed is a method and apparatus for uniformly planarizing a wafer that avoids the problems of the prior art. The solution needs to provide flexibility to the planarization process to correct for nonuniform polishing, while remaining simple and cost-effective.
The present invention is an apparatus and method for controlling the distribution of slurry across a polishing pad during a chemical mechanical polishing process. The invention allows the removal rate of material from different areas on the front surface of the wafer to be improved by adjusting the distribution of slurry across the polishing pad. Adjustments may be made before or during the planarization process. An object of the invention is to provide a method and apparatus that may be used to alter the removal rate of material from the front surface of the wafer to compensate for nonuniform planarization results. Another object of the invention is for the invention to be simple and inexpensive while avoiding the problems of the prior art.
The apparatus includes a slurry distribution system for controlling the distribution of slurry on a top surface of a polishing pad in a polishing station. The polishing station is used to planarize the front surface of a wafer. The slurry distribution system includes a polishing pad supported by a perforated platen. The polishing pad and platen may have aligned perforations to facilitate the transportation of slurry through them. A perforated top manifold may be positioned beneath the platen. The perforated top manifold may be juxtaposed with the platen, but a small gap preferably exists between the platen and the top manifold thereby creating a smoothing plenum. A perforated bottom manifold may be juxtaposed with the top manifold. A slurry chamber defining a slurry reservoir may be positioned beneath the bottom manifold.
The top and bottom perforated manifolds may be used to control the distribution of slurry across the polishing pad. By moving either the top or bottom manifold by a motor, a different pattern of aligned perforations may be created. By creating a pattern having a desired concentration of perforations in particular areas across the surface of the polishing pad, a desired concentration of slurry may be distributed to each area. The shape, size, position, and relationship of the perforations in the top and bottom manifolds may be selected to assist in making a wide range of adjustments to the slurry distribution across the polishing pad.
A slurry tank may be used for holding slurry for one or more chemical mechanical polishing tools. A pump may be used to communicate slurry from the slurry tank along a fluid communication path to the slurry reservoir. The pump, possibly in combination with one or more valves in the fluid communication path, may be used to control the volume of fluid delivered to the top surface of the polishing pad.
In a preferred embodiment, the platen is connected to an orbital motion generator. Orbital motion of the polishing pad during the planarization process with the described slurry delivery system is desirable (but not mandatory) for several reasons. Polishing pads used on orbital polishing stations tend to be smaller than on other types of polishing stations and are typically only slightly larger than the wafer. Smaller polishing pads make it easier to match areas on the front surface of the wafer that correspond with the areas on the polishing pad that they are polished against. However, other types of polishing stations, e.g. linear, rotational, etc., may also be used.
In operation, a desired distribution of slurry across a polishing pad may be achieved by moving, preferably rotating, either a perforated top manifold or a perforated bottom manifold to create a desired pattern of aligned perforations. Slurry may be transported from a slurry tank through the aligned perforations in the top and bottom manifolds to a top surface of the polishing pad. By moving the manifolds in relation to each other, a different pattern of aligned perforations may be created having different concentration of perforations. Areas on the polishing pad above areas of the manifolds having more perforations will have greater slurry flow than areas on the polishing pad above areas of the manifolds having fewer perforations. Applicant has noticed that areas on the polishing pad having greater slurry flow will produce faster material removal rates on the front surface of the wafer.
In another embodiment of the invention, a metrology instrument may be used to measure the surface of the wafer either during or after the planarization process. Metrology instruments, for example end-point detection systems, are known in the art. If measurements are taken during the planarization process, the slurry distribution across the polishing pad may be altered during the planarization process to correct for nonuniform planarization of the wafer. That is, areas on the polishing pad in contact with areas on the wafer polishing too quickly/slowly may receive less/more slurry. In determining this improved slurry distribution, many factors, such as the type of slurry, type of polishing pad, and material on the front surface of the wafer being planarized will need to be considered. In addition, the downforce and relative motion between the wafer and the polishing pad may also need to be considered in determining the improved slurry distribution. Computer modeling and empirical methods may be used to predict the improved slurry distribution needed based on these factors.
The results of the measurements taken during the planarization process may also be used to adjust the initial slurry distribution for the next wafer to be planarized. Measurements may also be taken by an inline or stand alone metrology instrument after the wafer has finished the planarization process. One advantage of waiting to take the measurements after the planarization process is that it is much easier to take measurements outside the harsh planarization process. Another advantage is that more time may be spent taking the measurements resulting in very accurate measurements. However, by taking the measurements after the planarization process, the results of the measurements will generally not be used for the benefit of the wafer being measured.